There has been conventionally known a variety of heat exchangers. For example, Patent Document 1 discloses a heat exchanger including a first fluid flow portion formed of a honeycomb structure having a plurality of cells to allow a heating medium as a first fluid to flow therein, and a second fluid flow portion located on an outer peripheral face of the first fluid flow portion. A coolant flows through the second fluid flow portion, taking heat from the heating medium flowing through the first fluid flow portion to cool the heating medium. Patent Document 1 also discloses layered honeycomb structures having gaps to allow the second fluid to flow therein.